Canis ISSN: 2398-2942

Ventricular fibrillation

Synonym(s): VF

Contributor(s): Dan Ohad, Jill Sammarco


  • Cause: can occur as electrophysiological complication of other "malignant" (ie prefibrillatory) ventricular arrhythmia such as very fast and sustained ventricular tachycardia (VT) in the presence of severe organic myocardial disease (eg sub-aortic stenosis or "Boxer dog cardiomyopathy").
    • In severe heart failure, when contractility is depressed and intraventricular diastolic filling pressure is high, VT can be a precursor to VF.
  • Can also occur as a consequence of death: untreated ventricular fibrillation → to hemodynamic cardiac arrest and death, ie often the result or final manifestation of death due to terminal cardiac or extra-cardiac disease.
  • May be induced by lengthy or deep-planed general anesthesia (eg when lengthy and severe myocardial hypoxia present) or at onset of anesthesia induction, either in severely diseased patients or (rarely) in healthy animals.
  • Respiratory arrest and ventilatory failure can lead to VF more commonly than does primary organic cardiac disease.
  • Diagnosis: electrocardiography. Chaotic/random, irregular and deformed, low-amplitude oscillations or undulations at a rate ranging between 150 and 300, or even >400 cycles/minute, with no distinct P waves (despite continuation of organized atrial electrical and mechanical activity), QRS complexes or T waves.
  • Undulations either coarse (and relatively more amenable for cardioversion) or fine (less amenable for cardioversion).
  • Many malignant ventricular arrhythmias that electrocardiographically and hemodynamically appear to be asystole (see Differential Diagnosis) are, in fact, fine VF. Therefore, open-chest heart massage and direct observation of myocardial activity may be warranted early with this arrhythmia.
  • Treatment: direct current (DC) electrical countershock (defibrillation) only effective therapy.
  • Prognosis: in the severely diseased, poor to grave, often even if promptly and successfully treated.
  • More favorable if defibrillation performed immediately in otherwise normal animal with chemical etiology eg drug toxicity:
    • Macrolide antibiotics like tylosin when given to animals with pre-existing myocardial ischemia.
    • Anesthetic overdose such as halothane or barbiturates.
    • Drug-drug interaction.


Predisposing factors


Primary cardiac disease

Extra-cardiac disease

  • Severe, terminal systemic disease (eg neoplastic/traumatic/endocrine).
  • Hyperkalemia (>11 mEq/L) (eg secondary to hypoadrenocorticism/"Addison's Disease" Hypoadrenocorticism ) or severe hypomagnesemia.
  • Severe acid-base imbalance Acid base imbalance (eg metabolic or respiratory alkalosis or acidosis).
  • Progressive (eg anemia Anemia: overview ) or acute (eg severe hemorrhage or prolonged ventilatory arrest) severe hypoxia/anoxia.
  • Extreme hypothermia Hypothermia or hyperthermia Hyperthermia.
  • A surge (or an iatrogenic bolus at a toxic dose) of circulating cathecholamines, especially when myocardium has been previously sensitized.
  • Acute, severe intoxication/drug toxicity (eg digoxin Digoxin ).
  • Electrocution/lightning stroke.
  • Thiamylal or halothane Halothane anesthesia of dogs following severe toad poisoning Toad poisoning that results in malignant ventricular arrhythmias.


  • Both ventricles "quiver" rather than contract, therefore stroke volume and cardiac output drop to zero even if some minimal, disorganized and non-coordinated myocardial mechanical activity still takes place.


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Further Reading


Refereed papers

Other sources of information

  • Crowe D T, Fox P R, Devey J J & Spreng D (1999) Cardiopulmonary and cerebral resuscitation. In: Textbook of Canine and Feline Cardiology. Principals and Clinical Practice. 2nd Edn. Eds: P R Fox, D Sisson D and N S Moise. WB Saunders Co, Philadelphia. pp 427-445.
  • Kittleson M D (1998) Diagnosis and treatment of arrhythmias (dysrhythmias). In: Small Animal Cardiovascular Medicine. Eds: M D Kittleson and R D Kienle. Mosby Inc, St Louis. pp 449-494.
  • Labato M A (1995) Cardiopulmonary arrest and resuscitation. In: Textbook of Veterinary Internal Medicine. diseases of the Dog and Cat. 1, Eds: S J Ettinger and E C Feldman. WB Saunders Co, Philadelphia. pp.71-79.
  • Lunney J & Ettinger S J (1995) Cardiac arrhythmias .In: Textbook of Veterinary Internal Medicine. diseases of the Dog and Cat. Eds: S J Ettinger and E C Feldman. WB Saunders Co, Philadelphia. pp 959-995.
  • Detweiler D K (1988) The dog electrocardiogram: a critical review. In: Comprehensive Electrocardiology. Theory and Practice in Health and Disease.2,Eds: P W Macfarlane and T D Lawrie. Pergamon Press, NY. pp 1267-1329.

Other sources of information

  • The Merck Veterinary Manual, Eighth Edition, Published by Merck & Co Inc, Whitehouse Station, NJ, USA. In cooperation with MERIAL LIMITED, a Merck and Aventis Company rights reserved).